Inconel Alloys – Inconel 600 – Inconel 625 – Inconel 718 – X-750

Inconel is a trade name of Special Metals Corporation. Other metal producers of general materials cannot use the Inconel trademark. The five more popular grades in the Inconel family are Inconel alloys 600, 601, 625, 718 and X-750. The first three cannot be hardened by heat treatment. The others are grades that can be strengthened by a “precipitation hardening” heat treatment.

Nickel and chromium are the main elements of these alloys, which provide strength, corrosion resistance and oxidation resistance in a wide range of temperatures and environments.

Alloys in this family have a variety of uses in chemical processing equipment and in many high temperature applications such as furnaces and retorts where oxidation resistance is required. Inconel alloys are found in aerospace applications including jet and rocket engine components.

Inconel 600 UNS designation N06600 has a nickel content of at least 72% (plus cobalt).

As we pointed out earlier, the two metals are metallurgically similar and are considered as one element in the composition. The alloy has a Cr content of 14-17% and a Fe content of 6-19%. The maximum content of carbon is limited to 15%, the maximum content of manganese is limited to 1.0%, the maximum content of sulfur is limited to 0.15%, and the maximum content of Si and Cu is limited to 50%.

This alloy cannot be hardened by heat treatment.

Inconel 601 UNS Designation N06601 Improved high temperature oxidation resistance due to the increase in Cr content to 21 to 25% and 1.0 to 1.7% Al. Ni (plus Co) is 58-63%. C is 10 points maximum, S is 0.15% maximum, Si is 50% maximum, and Mn and Cu are each 1.0% maximum. The rest is Fe.

This alloy cannot be hardened by heat treatment either.

Inconel 625 UNS N06625 For higher strength applications. It is strengthened by the addition of Mo (Molybdenum) and Cb (Niobium). Mo also helps to resist corrosion and high temperature oxidation. The composition is at least 58% Ni (plus Cobalt), 20% to 23% Cr, 8% to 10% Mo and 3.15% to 4.15% Niobium (Cb) (plus Ta (Tantalum)). In this application, the elements Niobium (Cb) and Tantalum (Ta) are considered as one of the constituent elements. Limiting elements allow a maximum %….. Fe 5.0%, Carbon 10%, Manganese 50%, Si 50%, Al 40%, Ti 40%, Phosphorus (P) 0.15% and Sulfur 0.15%.

Some specifications limit any Co content to a maximum of 1.0 %

This alloy cannot be hardened by heat treatment.

Inconel 718 UNS N07718 is a high strength alloy with useful properties in the range of -423 to 1300 degrees Fahrenheit. The composition calls for 50 to 55% Ni (plus Co), 17 to 21% Chromium, 4.75 to 5.50% Niobium + Tantalum Cb (plus Ta), 2.80 to 3.30% Mo, 65 to 1.15% Ti, 20 to 80% Al.

The limiting elements and the maximum values allowed for each are C 0.8%, Mn 0.35%, Si 0.35%, P 0.15%, S 0.15%, Boron 0.06% Cu 0.30%, Co 1.0%. Fe is considered the balance. This alloy can be precipitation hardened. Solution treatment is usually done between 1750 and 1950 degrees Fahrenheit, followed by aging at 1325 degrees Fahrenheit for 8 hours, then slow cooling to 1150 degrees Fahrenheit and holding for 8 hours.

Inconel X-750   UNS N07750 Composition is at least 70% Ni (plus Co), 14 to 17% Cr, 5 to 9% Fe, 2.25 to 2.75% Ti, 4th to 7% Aluminum, and 7th to 1.20% Niobium + Tantalum Cb (plus Ta). Limiting elements are Mn to 1.0%, Si to 5%, Cu to 5%, S to 01%, C to 08%, and Co to 1.0%

This alloy responds to precipitation heat treatment, which is usually done in three steps. Solution heat treatment is followed by intermediate aging and then final aging. The time and temperature depend on the characteristics of the specific application.